1. Field of the Invention
The present invention relates to position control systems of actuators with brush-type permanent magnet DC motors and, more particularly, to actuator position control systems utilizing commutation pulse feedback.
2. Description of the Related Art
The feedback signals of actuator position control systems are normally achieved by using encoders, resolvers, or potentiometers. More recently, commutation pulses embedded in the current of brush-type permanent magnet DC motors have been used as feedback signals (referred to as xe2x80x9cpulse countxe2x80x9d).
Encoders and resolvers can provide good accuracy and reliability but they are too expensive for applications like HVAC control in automobiles. Potentiometers have been a popular choice for such low cost applications.
The typical arrangement for a potentiometer actuator includes a brush-type permanent magnet DC motor, and a gear train to increase the output torque. The potentiometer is driven either directly by the output shaft (wiper and carbon traces), or through another gear driven ba the output gear. The output voltage as measured from the center tap of the potentiometer is then used to determine the output shaft position.
Due to the mechanical contact (wiper and carbon traces), the accuracy, stability, and reliability of such potentiometer systems is significantly compromised. In addition, the cost of such systems is still relatively high.
Pulse counting is done through monitoring of DC motor current signals. A sensor is connected to the motor power line to sense the current ripples due to commutation and, with a fixed design, the number of the ripples per shaft revolution is a constant. As a result, the motor shaft position can be determined accurately according to the pulse counting result.
Resistors can be used as sensors for current signal detection via different kinds of connections to the motor power lines. However, due to the high variation in current profile, as well as the environment and life of the systems, the accuracy and reliability of such sensor systems may be adversely affected. Furthermore, the sensing resistor also results in poor system efficiency, which means a bigger actuator is needed for the same application because the resistance must be large enough for sufficient signal sensing.
Therefore, accuracy and reliability have become obstacles to development of actuator position control systems, especially for control systems in automotive applications. Accordingly, a low cost solution with a simple circuit and high system efficiency is desirable.
In view of the foregoing, one object of the present invention is to overcome the difficulties of complexity and cost found in prior art actuator position control systems.
Another object of the invention is a pulse count system for brush-type DC motors that uses inductive sensing for clean, accurate, and reliable shaft position signals to provide accurate actuator position control.
A further object of the invention is an inductive sensing actuator position control system that exhibits high system efficiency, smooth actuator operation, and insensitivity to motor life and environmental temperature change.
A still further object of the invention is a simple circuit design for actuator position control that provides a highly reliable system at low cost.
In accordance with these and other objects, the present invention is directed to an actuator position control system for brush-type DC motors. The system includes a signal sensor, an amplifier, a central control unit such as a microprocessor, a motor driver and a permanent magnet DC motor. An inductor is selected as the signal sensor, because voltage drops across the inductor are directly proportional to di/dt. The voltage drop, L(di/dt), across the inductive sensor is input to the amplifier. The output pulses from the amplifier are sent to the microprocessor to determine output shaft position, and speed control can also be achieved using the output pulse information.
These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.